The present invention relates to a wheel alignment measuring apparatus in accordance with the generic part of claim 1.
For a long time it has been common practice in the vehicle industry to measure and adjust the individual track of the front wheels relative to the geometrical driving axis.
It is well known that the geometrical driving axis is defined as the bisecting line from the total track of the rear axle and equals the imaginary line analogously formed from the angular position of the two rear wheels in the longitudinal direction of the vehicle. A toe-in "0" means a position where the wheel axle of one of the front wheels is at right angles to the geometrical driving axis.
In particular wheel alignment measuring apparatuses for trucks and buses should also be able to measure the above parameters with a high precision and, in addition to that, should be easily operable. In the case of buses this is caused by the high safety requirements with respect to the passengers, which should therefore particularly take into account the high speeds driven on motorways.
Furthermore, in the case of trucks provided with two steerable front axles the correspondence of the positions of all wheels with respect to each other is of special importance. Otherwise, in the case of an almost identical load acting on the wheels an inexact adjustment of the wheels with respect to each other would result in an unusually high wear of the tires. In view of the costs of such a set of tires a high mileage is particularly desirable in the case of trucks.
The technical preconditions for the measurements of the wheel alignment in the case of trucks and buses are, due to their weight and dimensions, different from those in motorcars, and therefore require a different system and a different design of the wheel alignment measuring apparatus.
Thus, a known method according to which a vehicle is placed in a given optical rectangle firmly connected with the underlying ground requires a considerable amount of space in the case of buses. In practice, the necessary alignment movement of the vehicle within the given rectangle requires an enormous amount of work due to the weight of the vehicles.
Methods using light rays for the determination of the wheel alignment are problematic due to the low wheel base. With increasing distance the light rays lose their luminous power, the light spots increase and in order to be able to read the values the operator will often have to change the place and the side. Due to legal provisions it is forbidden to replace the light rays by laser rays.
In another known measuring method units provided with arms are mounted at the front wheels, said arms being parallel to the front wheels and provided with rotary potentiometers at their ends. The tension arms of the rotary potentiometers are connected with each other by means of elastic elements and thus help to determine the angular position of the tension arms with respect to each other. This measuring method, which has proved quite successful in the measurement of motorcars, is difficult to apply in the case of buses and trucks. The difficulties result from the length and depth of the front parts of the buses. Measured from the middle of the axle of the front wheels the length of the front part is 2 to 3 m. The arms supporting the angle pick-ups in this measuring method abut the front part during a swivel movement of the wheels which is necessary for the measurement of the caster and the toe-in and is 20.degree. on each side. Further difficulties arise from the length of the distance between the front wheels and the rear wheels since at this length tension elements tend to vibrate.
It is thus an object of the invention to create a wheel alignment measuring apparatus particularly suited for trucks or buses, which can function without units mounted at the front wheels and provided with arms, is thus extremely space-saving and, moreover, provides very accurate results and can, at the same time, be easily handled.
This object of the invention is solved in a wheel alignment measuring apparatus in accordance with the present invention.
The present invention can also be advantageously applied in trucks with two steered front axles since the position of the four front wheels is measured at the same time from a base, which so far has not been considered to be possible. The invention is of course not restricted to trucks or buses. It can rather be very successfully used in motorcars as well.